Voila
pourquoi les laves ressemblent tellement aux pierres naturelles des
especes analogues, qu'elles ne peuvent en etre distinguees; voila
egalement pourquoi les verres volcaniques eux-meme renferment encore des
substances elastiques qui les font boursoufler lorsque nous les fondons
de nouveau, et pourquoi ces verres blanchissent aussi, pour lors, par
la dissipation, d'une substance grasse qui a resiste a la chaleur des
volcans, et que volatilise la chaleur par laquelle nous obtenons leur
second fusion."
No doubt, the long application of heat may produce changes in bodies
very different from those which are occasioned by the sudden application
of a more intense heat; but still there must be sufficient intensity in
that power, so as to cause fluidity, without which no chemical change
can be produced in bodies. The essential difference, however, between
the natural heat of the mineral regions, and that which we excite upon
the surface of the earth, consists in this; that nature applies heat
under circumstances which we are not able to imitate, that is, under
such compression as shall prevent the decomposition of the constituent
substances, by the separation of the more volatile from the more fixed
parts.
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